The development of downhole instrumentation to evaluate drilling and coring of earth formations, has been given impetus by various governmental committees and councils. Prognosis: While instrumentation and uses involving measurements while drilling (or "MWD"), are well-documented, gains to be obtained from measurements while coring (or "MWC") have not yet crystallized. Reasons: Many of most difficult well control problems occur when a core barrel is the well bore. Not only is the ability to handle well kicks reduced (because of reduced circulation capability) but there is increased likelihood of plugging and jamming.
That is to say, the benefits to be gained from MWC during exploratory coring have not been documented in sufficient fashion to outweigh the safety concerns of the field operators. Moreover, the type of real-time data desired or justified, is subject to speculation.
In our prior application, op. cit., we describe use of a single measurement means mounted adjacent to the uphole terminus of the inner core barrel to monitor rotation, if any, of the inner barrel. Such rotation is surprisingly indicative of core twist-off or core sand erosion as the core is being extracted. One element of the measuring means can be a Hall-effect device powered by electrical energy generated by a generator and mud turbine housed at a drill string segment, slightly uphole from the Hall-effect device.
When the time allowed for performing coring operations is long, electrical powering of the Hall-effect device, as previously described, has proven to be quite useful. But for shorter--timewise--coring situations, operations, using energy generated by downhole battery means, have proven to be surprisingly efficient.